1. Field of the Invention
The present invention relates to a semipermeable membrane. More particularly, the present invention is concerned with a surface-hydrophilic, highly selective semipermeable membrane which has not only excellent resistance to heat and organic solvents, but also non-adsorptivity for organic substances. The present invention is also concerned with a process for producing the surface-hydrophilic, highly selective semipermeable membrane having non-adsorptivity for organic substances. The semipermeable membrane of the present invention can advantageously be used for microfiltration, ultrafiltration, reverse osmosis and dialysis.
2. Discussion of Related Art
Various attempts were made to produce a semipermeable membrane having excellent heat resistance and a high selectivity.
Recently, an attempt was made to introduce a group having an electric charge to the surface of a semipermeable membrane comprised of a hydrophobic engineering plastic having a heat resistance, thereby rendering the semipermeable membrane hydrophilic. The surface-modified semipermeable membrane is relatively stable in a solution of a temperature as high as 80.degree. to 100 .degree. C with respect to properties, such as separating characteristics and permeability, since the hydrophobic backbone structure of the engineering plastic constitutes a three dimensional skeleton of the semipermeable membrane. Heretofore, to introduce a group having an electric charge to the surface of an engineering plastic, a sulfonation reaction method, in which a sulfonic group is directly introduced to an aromatic ring of the engineering plastic, has mainly been used. A sulfonated engineering plastic can easily be synthesized and, therefore, it is often used as a material for an anionic semipermeable membrane. A representative sulfonated polysulfone is disclosed in, for example, U.S. Pat. No. 3,709,841.
There is also known a surface-sulfonated polysulfone type or surface-sulfonated polyether-imide type semipermeable membrane, which is obtained by treating a polysulfone type or polyether-imide type semipermeable membrane with a sulfonating agent. However, it is noted that since the sulfonation is a reversible reaction (see, R.T. Morrison and R.N. Boyd, "Organic Chemistry", the third edition, volume 1 (1977), p.437-442, published by Tokyo Kagaku Dojin K.K., Japan), desulfonation of a sulfonated polymer disadvantageously occurs when the sulfonated polymer is exposed to an aqueous acid solution at a high temperature. Further, it is presumed that in the case in which a sulfonic group is introduced directly to the aromatic ring of the polymer skeleton of an engineering plastic, the movement of the group having an electric charge (i.e., sulfonic group) is extremely inhibited by the action of the polymer skeleton so that the effective electric charge density becomes low.
Further, it is known that a sulfonated resin having a high sulfonation degree, that is, having a high ion-exchange capacity, is characterized with poor resistance to organic solvents and, therefore, the sulfonated resin is likely to be dissolved in a mixture of acetone and water or a mixture of alcohol and water (see, Japanese Patent Application Publication Specification No. 53-32840). To solve this problem, some studies were made. For example, in Japanese Patent Application Laid-Open Specification No. 62-269704, it is disclosed that a sulfonated polysulfone membrane comprising a polysulfone and a sulfonic group bonded indirectly thereto through a methylene group, which is produced by treating the surface of a polysulfone with propanesultone in the presence of a Friedel-Crafts catalyst. In this Laid-Open Specification, it is described that the thus treated polysulfone is improved with respect not only to having a resistance to heat and organic solvents, but also to the effective charge density. This is due to an indirect bonding of a sulfonic group to the polysulfone through a joint group (methylene group).
In this connection, however, it is noted that there is another problem with respect to a membrane having an electric charge. The problem is that a group having an electric charge present in the membrane is capable of binding to a counter ion. When ions of organic substances having a high molecular weight, such as a surfactant, are contained as a counter ion in a solution to be contacted with the membrane, the membrane is likely to adsorb organic substances, which causes the fouling of the membrane and the lowering of the flux of the solution. Even in the case of the membrane having a sulfonic group bonded indirectly thereto through a joint group as described in Japanese Patent Application Laid-Open Specification No. 62-269704, the above-mentioned problem remains unsolved.